This invention relates to methods and apparatus for ion implantation of a workpiece and, more particularly, to a beamline architecture which permits ion implantation of semiconductor wafers with low energy ions.
Ion implantation has become a standard technique for introducing conductivity-altering impurities into semiconductor wafers. A desired impurity material is ionized in an ion source, the ions are accelerated to form an ion beam of prescribed energy, and the ion beam is directed at the surface of the wafer. The energetic ions in the beam penetrate into the bulk of the semiconductor material and are embedded into the crystalline lattice of the semiconductor material to form a region of desired conductivity.
Ion implantation systems usually include an ion source for converting a gas or solid material into a well-defined ion beam. The ion beam is mass analyzed to eliminate undesired ion species, is accelerated to a desired energy, and is directed onto a target plane. The beam is distributed over the target area by beam scanning, by target movement or by a combination of beam scanning and target movement. Examples of prior art ion implanter are disclosed in U.S. Pat. No. 4,276,477 issued Jun. 30, 1981 to Enge; U.S. Pat. No. 4,283,631 issued Aug. 11, 1981 to Turner; U.S. Pat. No. 4,899,059 issued Feb. 6, 1990 to Freytsis et al; and U.S. Pat. No. 4,922,106 issued May 1, 1990 to Berrian et al.
A well-known trend in the semiconductor industry is toward smaller, higher speed devices. In particular, both the lateral dimensions and the depths of features in semiconductor devices are decreasing. State of the art semiconductor devices require junction depths less than 1000 angstroms and may eventually require junction depths on the order of 200 angstroms or less.
The implanted depth of the dopant material is determined, at least in part, by the energy of the ions implanted into the semiconductor wafer. Shallow junctions are obtained with low implant energies. Ion implanters are typically designed for efficient operation at relatively high implant energies, for example, in the range of 50 KeV to 400 KeV, and may not function efficiently at the energies required for shallow junction implantation. At low implant energies, such as energies of 2 KeV and lower, the-current delivered to the wafer is much lower than desired, and in some cases may be near zero. As a result, extremely long implant times are required to achieve a specified dose, and throughput is adversely affected. Such reduction in throughput increases fabrication cost and is unacceptable to semiconductor device manufacturers.
An ion implanter configuration for producing a high current ribbon beam is disclosed in U.S. Pat. No. 5,350,926 issued Sep. 27, 1994 to White et al. An ion source generates an ion beam that diverges in a horizontal plane. An analyzing magnet deflects a desired species in the ion beam to a resolving slit and focuses the desired species in the ion beam on the resolving slit. A second magnet deflects the beam that passes through the resolving slit to produce parallel ion trajectories. The disclosed ion implanter has highly satisfactory performance under a variety of conditions. However at low energies, space charge effects cause the beam to expand, particularly in the high current density region where the beam is focused, and performance at low energies may be unsatisfactory. Another beamline architecture having similar deficiencies is disclosed in U.S. Pat. No. 5,126,575 issued Jun. 30, 1992 to White.
Accordingly, there is a need for improved beamline architectures for ion implanters, which are capable of delivering high ion beam currents at low energies.
According to a first aspect of the invention, an ion beam apparatus is provided. The ion beam apparatus comprises an ion source, a first magnet assembly, a structure defining a resolving aperture and a second magnet assembly. The ion source has an elongated extraction aperture for generating a ribbon ion beam. The first magnet assembly provides first magnetic fields for deflecting the ribbon ion beam perpendicular to the long dimension of the ribbon ion beam cross section, wherein different ion species in the ribbon ion beam are separated. The resolving aperture selects an ion species from the separated ion beam. The second magnet assembly provides second magnetic fields for deflecting ions of the selected ion species in the ribbon ion beam parallel to the long dimension of the ribbon ion beam cross section to produce desired ion trajectories of the selected ion species in the ribbon ion beam. Preferably, the desired ion trajectories are substantially parallel. The width of the ribbon ion beam increases through most of the beamline. As a result, low energy performance is enhanced.
The first magnet assembly preferably comprises a resolving magnet having first polepieces separated by a first gap through which the ribbon ion beam passes. The second magnet assembly preferably comprises an angle corrector magnet having second polepieces separated by a second gap through which ions of the selected ion species in the ribbon ion beam pass. In a preferred embodiment, the first magnetic fields produced by the first magnet assembly are substantially horizontal and the second magnetic fields produced by the second magnet assembly are substantially vertical. The first magnet assembly preferably deflects ions of the selected ion species in the ribbon ion beam by an angle in a range of about 20xc2x0 to 90xc2x0 and more preferably by an angle of about 60xc2x0. The second magnet assembly preferably deflects ions of the selected ion species in the ribbon ion beam by an angle in a range of about 20xc2x0 to 90xc2x0 and more preferably by an angle of about 70xc2x0.
In one embodiment, the structure defining the resolving aperture comprises a mask positioned between the first and second magnet assemblies. In another embodiment, the structure defining the resolving aperture comprises the second polepieces of the second magnet assembly, and an entrance to the second gap constitutes the resolving aperture.
According to another aspect of the invention, a method is provided for producing an ion beam. The method comprises the steps of generating a ribbon ion beam in an ion source having an elongated extraction aperture, deflecting the ribbon ion beam perpendicular to the long dimension of the ribbon ion beam cross section, wherein different ion species in the ribbon ion beam are separated, selecting an ion species from the separated ribbon ion beam, and deflecting ions of the selected ion species in the ribbon ion beam parallel to the long dimension of the ribbon ion beam to produce desired ion trajectories of the selected ion species in the ribbon ion beam.